1. Field of the Invention
The present invention relates to a magneto-optical recording medium and magneto-optical recording method wherein the recording, erasure, and reproduction of data is magnetically accomplished in a heated state by means of irradiation with a laser beam.
2. Description of Related Art
Recently, optical recording and reproduction methods have been developed that satisfy demands for high density, high capacity, high access speed, high recording speed, high reproduction speed, etc. In addition, recording devices in the form of reproduction devices and recording mediums have been developed using the recently-developed recording and reproduction methods for these stated purposes.
Among the many optical reproduction methods, the magneto-optical method is capable of over-write operation. That is, the magneto-optical method is capable of erasing data that has already been recorded on the recording medium. Following erasure, the magneto-optical method has the advantage of being capable of again recording new data on the recording medium, a capability that has a great deal of appeal.
Generally, the magneto-optical method is accomplished through the irradiation of a laser beam on a medium that possesses a perpendicularly magnetized film recording layer. The direction of magnetization of the recording layer is predirected either upwardly or downwardly, as the case may be, in advance of recording. When recording, a portion of the recording layer is heated to a temperature close to that of the Curie point, at which magnetization becomes zero, by means of a laser beam having a very small diameter of about 1 .mu.m. This small diameter laser beam forms a mark that has an opposite direction of magnetization by reducing the coercivity (Hc) of the locally heated portion of the recording medium to a value below the magnetic recording bias field (Hb). The data is manifest on the recording medium by means of the existence or nonexistence of, and the length of, the resulting mark. Reproduction of the recorded data is accomplished by irradiating polarized light beams onto the recording layer of the recording medium. The polarized light is rotated by .theta..sub.K degrees in the recording layer according to the direction of magnetization, a principle known as the magnetic Kerr effect. The reflected light is then detected, analyzed and converted to an electric signal, thus reading the data.
The recording medium that utilizes this magneto-optical recording and reproduction method may utilize, as the recording layer, known amorphous alloys of rare earth and transition metals that possess perpendicular magnetic anisotropy. In particular, TbFeCo is generally used because it is a substance in which the recording state can be stably maintained, and it has large coercivity.
Various methods exist that can be used as the method for forming a recording film, including the evaporation method, the ion plating method, the sputtering method, and the ion beam sputtering method. However, general use is made of the sputtering method, in which utilization is made of argon gas due to its known ease of handling. Since there is efficiency in the ionization of the argon gas in this method, the magnetron sputtering method is generally used, in which a magnet is housed at the bottom of the sputtering target. In addition, the magnetron sputtering method uses multiple targets, comprised of rare earth metal targets and transition metal targets. These respective targets allow for the utilization of such known methods as the simultaneous plural sputtering method, which independently controls the sputtering, and the sputtering method in which alloy targets are used that comprise rare earth alloy metals and transition metals.
However, problems are noted if the magneto-optical recording medium is constructed by means of the magnetron sputtering method using the above-mentioned simultaneous plural sputtering procedure. Namely, because rare earth metal targets and transition metal targets are used in the simultaneous plural sputtering, compositional changes in terms of thickness are small. However, use of the magnetron sputtering method, combined with the dissipation of the transition metal target, causes the sputtering efficiency to be changed by means of changes in the stray field from the magnet that is located at the bottom of the target. As a result, there is a complete change in the proportional ratio between the rare earth metals and the transition metals over the passage of time. In other words, there is a complete change in the composition.
On the other hand, the target which is an alloy of rare earth metal and transition metal changes in its composition in terms of thickness during the manufacturing method. As a result, in accompaniment with the dissipation of the target, there is the problem of a complete change in the composition of the formed film.
The existing problem, therefore, is that there is a change in recording sensitivity of the magneto-optical recording medium. This change is striking, particularly when a TbFeCo recording film is used.